Nanoimprint lithography is a well-known technique for fabricating nanometer scale relief patterns using patterned templates, which are also called masks or molds. The templates are used for imprinting a pattern into a suitable material, such as a liquid resist. The material may then solidify while still in contact with the template, which is removed after the solidification. When removing the template, particles may adhere to the template surface. During the nanoimprint lithography process, the template comes into direct contact with the resist on the substrate. Therefore, template cleanliness plays an important role in imprinted substrate quality. Furthermore, if the template has any form of a defect such as resist residue, stains, particles, surface scratches, chipping, bumping, etc., it can lead to poor quality imprints, low yield and throughput decreases. Hence, any periodic cleaning of the templates should preferably be efficient in removing contaminants and should avoid any damages to the surface of the templates.
In the past, chemical cleaning strategies have been used, which may be problematic as discussed in S. Singh et al. “Effects of Cleaning on NIL Templates: Surface Roughness, CD and Pattern Integrity” in the Proc. of SPIE Vol. 8166 81662P-1, or Zhang et al in “Mask cleaning in EUV and Nano-Imprint Lithography” China Semiconductor Technology International Conference 2010, ECS Transactions 27(1) 467-472.
EP 2 056 164 A1 describes a cleaning apparatus using plasma radicals for the cleaning of substrates in an immersion lithographic apparatus. In particular, atmospheric plasma cleaning is described in which plasma radicals are generated in a plasma radical source and then directed in a gas flow, via a conduit, to an outlet which is directed towards a contaminated surface. To restrict the plasma radicals to a localized region of the surface to be cleaned, one or more suction openings are provided adjacent the outlet. The suction openings are connected to an under-pressure source to exhaust gases and radicals directed onto the surface to be cleaned via the outlet together with any gases which are drawn into the suction openings from the immediate surroundings thereof.
Further examples of apparatuses and methods using plasma excited gases for cleaning purposes are shown in U.S. Pat. No. 2,015,270 119 A1 or US 2009 186 167.